1. Field of the Invention
The present invention relates to a glow plug fitted with a combustion pressure detection sensor including a function of promoting ignition in a combustion chamber of an engine and additionally sensing (detecting) the combustion pressure.
2. Description of the Related Art
Various glow plugs including a combustion pressure detection sensor (hereinafter, also referred to as a glow plug) have been known (see, for example, Patent Document 1). FIG. 8 is a partial longitudinal cross-sectional view that shows a simplified cross-sectional structure of this kind of glow plug. The glow plug 901 of FIG. 8 includes a rod-like (a circular rod shape) heater (for example, a sheath heater or a ceramic heater) 10 generating heat by applying voltage to a housing 110 forming a cylindrical shape. A leading end (an end of a side protruding into the combustion chamber, not shown) 10a of the heater protrudes from a leading end 136 of the housing 110. In addition, “a two-dot chain line” in FIGS. 8 and 9 shows a portion in the vicinity of a combustion chamber N in an engine head.
Since in the glow plug 901, the combustion pressure is detected based on displacements of the heater 10 due to the combustion pressure and a center pole (a pole member) or the like which is fixed thereto and extended rearward due to the combustion pressure or based on pressure applied to the sensor as a result of displacement of the center pole or the like, the heater 10 is placed in the housing 110 so as to be slightly displaceable in the front and rear direction thereof (a axial direction G). For this reason, the heater 10 is placed so as to maintain a gap (an annular gap) with an inner peripheral surface of the housing 110. Further, as a detecting means for detecting the displacement generated in the heater 10 or the like, for example, a piezo-electric element 40 is placed behind the heater 10. The glow plug is configured so as to compress the piezo-electric element 40 by force pressing the heater 10 rearward by the combustion pressure and displacing the heater 10. The combustion pressure is detected by measuring an electric signal generated according to a change in the compression force. In addition, the glow plug of Patent Document 1 is configured so that a strain gauge is adopted as the detecting means. By deforming a distortion member with displacement of the heater, the combustion pressure is detected using a sensor (a strain sensor) installed therein, based on the amount of deformation.
However, in the glow plug 901 including the combustion pressure detection sensor, there is a need to prevent (by securing with a seal) combustion gas of high temperature and high pressure from entering the housing 110 rearward from a leading end 136 of the housing 110 through an annular gap between the inner peripheral surface thereof and an outer peripheral surface of the heater 10. For this reason, in the glow plug 901 of FIG. 8, as shown in an enlarged views of FIG. 8 and FIG. 9, in a relatively expanded-diameter annular space K2 behind a leading end side annular gap K1 provided between the inner peripheral surface of a leading end part 131 of the housing 110 and the outer peripheral surface of the heater 10, a seal member 60 is provided. In this arrangement, the front and rear of the expanded-diameter annular space K2 is blocked in an airtight state.
Since the seal member 60 needs to allow for displacement of the heater 10 in the axial direction G relative to the housing 110, the seal member 60 is formed of a heat resistant material having a sufficiently resilient annular film portion (a membrane) 63. This may be a thin metal film (for example, a thin film made from SUS 630) which itself is capable of being deformed in a frontward and rearward direction (the axial direction G) such as a bellows or a diaphragm that is formed in a deformable manner. The seal member 60 of FIG. 8 has an annular film portion 63 so as to partition the front and rear of the expanded-diameter annular space K2 between a small-diameter cylinder portion 65 forming a cylindrical portion of a leading end side and a large-diameter cylinder portion 61 of a rear end side. In the expanded-diameter annular space K2, the small-diameter cylinder portion 65 is adhered along an outer peripheral surface of the heater 10, and is welded (laser welded) the outer peripheral surface of the heater 10 at a predetermined location (a triangle portion shaded black) W1. Moreover, the large-diameter cylinder portion 61 is fixed to the housing 110 side along a circumferential direction at a predetermined location (triangle portion shaded black) of the rear end side thereof at W2 and W3 by welding. Thus, sealing in the forward and rearward direction is secured between the inner peripheral surface of the part near the leading end of housing 110 and the outer peripheral surface of the heater 10.
The glow plug 901 mentioned above is screwed into a plug hole of the engine head, and the leading end 10a of the heater 10 is installed so as to be exposed to the inside of the combustion chamber. As a result, the combustion gas pressurizes the entire exposed heater 10, and enters the expanded-diameter annular space K2 via the leading end side annular gap K1 between the inner peripheral surface of the leading end part 131 of the housing 110 and the outer peripheral surface of the heater 10 to pressurize a leading end facing surface (mainly, the annular film portion 63) of the seal member 60 therein. Thus, the heater 10 is displaced so as to be pressed rearward based on an action of deforming the seal member 60 rearward due to the combustion pressure (herein, referred to as pressure) received by a leading end facing surface of the seal member 60 provided in the expanded-diameter annular space K2, in addition to the combustion pressure received in the leading end 10a of the heater itself.
That is, in the glow plug 901 of the structure mentioned above, the action in which the seal member 60 itself is deformed rearward contributes to displacing the heater 10 rearward or compressing the piezo-electric element 40, and the detection of the combustion pressure is performed based on this action. That is, in the glow plug, since the heater 10 is displaced rearward by the combustion pressure received by the heater itself and the seal member 60, the pressure of the combustion gas received by the seal member 60 greatly affects the detection accuracy of the combustion pressure.
[Patent Document 1] JP-A-2010-139148